Acceptability of "I'm trying to think if..." [duplicate]

Are if and whether equivalent in sentences like the ones below?

How to determine if my saddle is too high?
How to determine whether my saddle is too high?

We should check if everything is okay now.
We should check whether everything is okay now.

In general, if you're turning a question into a noun, whether tends to be preferred in formal use. So this includes your example plus, e.g.:

The discussion was about [whether climate change was an important issue].

The issue is [whether we need to act now].

In informal usage, if is also possible in these cases.

It's important to understand that 'whether' and 'if' are not always interchangeable. The word whether only has the meaning of "if ... or not" (though you can still use the formula whether ... or not for emphasis), so for example you can't use whether in this case:

If/*whether you decide to come, give me a call.

Another key difference is that whether can introduce an infinitive, whereas if can't (they're different parts of speech):

They couldn't decide whether/*if to come.

Whether is used to emphasis that one has two alternatives to pick from. Merriam-Webster gives two definitions:

1. archaic : which one of the two 
2. archaic : whichever one of the two

In the saddle example, there are three alternative results: too high, too low or just perfect. It doesn't make much sense to check the saddle for being too high and not caring about too low.

Let's make it simple and clear:

Whether - use this word for indicating 2 options

If - use this word to establish a condition for an action.

Informally, "if" is sometimes used when "whether" is the better word, and in many cases this doesn't lead to confusion. This means that, informally, "if" is used in more than one way. However, "whether" only has one meaning, so using "whether" when indicating options is preferred.

THUS:

1: How to determine whether my saddle is too high. (as in whether the saddle is too high or not--two options)

2: We should check whether everything is okay now. (as in whether everything is okay or not--two options)

Note: In both examples, the 2nd part of the expression "whether...or not" is implied. This is quite common and quite acceptable when doing so doesn't reduce clarity. This leads us to the next point.

You can correctly say "Whether you decide to come or not, give me a call." The shortened form, with the implied part 2, is "Whether you decide to come, give me a call." (I wouldn't write this because it reduces clarity. I would use the complete expression.)

One last comment: "Whether" doesn't need to be followed by a stated or implied "or not." The 2nd option could be any thing, as in "I don't know whether to buy cake or pickles."